The present invention relates to a screw with an improved head, in which a stress-relieving notch is disposed at each corner of a driving recess on the screw head.
As screws can be easily threaded into or removed from a workpiece to facilitate an assembling or disassembling job, they have been widely used nowadays. In particular, the advance of technology has made the screws possible for mass production, resulting in a cutdown in their manufacturing costs, and which is further promoting their use.
A screw generally comprises a head being larger in diameter and a threaded shank axially extending from the head. A driving recess is formed on the head at the center thereof for insertion of a tool to apply driving force. The driving recess can be designed to be different in shape. Apart from the well known cross, triangle, square, hexagonal, or multiangular driving recess is also available. In use, a screw driver with a corresponding cross section is inserted into the driving recess for applying driving force to turn the screw for tightening or removal purpose.
Although the screw of this type can meet its basic function requirements, it has the disadvantage that, upon the screw forge-forming process, material hardening treatment and stress concentration often cause the corners of the driving recess on the screw head to crack as shown in FIG. 1 crack A, thereby weakening the capability of such a driving recess to bear against the stress. On the other hand, the punch used for forge-forming purpose are also likely to get broken or worn out at the edge of the punch where an sharp edge exists. Consequently, it is difficult to obtain a good quality control in the course of mass production. Moreover, due to this disadvantage, only some materials can be selected for forge-forming purpose, it is difficult to use highly hardened materials, such as stainless steel, high carbon steel or aerospace Alloy.
It is the object of the present invention to provide a screw with an improved head, in which a stress-relieving notch is disposed at each comer of a driving recess on the screw head for distributing the stress in an attempt to prevent cracking as frequently occurred in a conventional screw.